There is known in the art to measure the level of the surface of a product stored in a tank by means of a radar level gauge. Such devices are disclosed in a number of documents. As an example of such a document, it will here be referred to patent document U.S. Pat. No. 4,665,403.
Radar level gauges for use, as an example within the processing industry, must be able to function under very different conditions. The product stored could be a lot of different products, such as petroleum refinery products, liquid gases and other chemical compounds. This implies that such parameters as temperature and pressure can be of very shifting values. Disturbing structures are also existing inside the tank. Such are, for example, devices as agitators, foam, etc., whereby measuring is rendered more difficult and may go wrong.
Three technical problems are recognized from this: 1° various structures inside the tank give disturbing radar echoes, 2° it is desirable to have an antenna creating a narrow antenna beam (among other to suppress disturbing echoes), 3° various propagation influences (dirt on the antenna, foam on a liquid product surface etc.) may reduce the desired echo greatly but in worst case leave the disturbing echoes. These problems are partly known in radar history (see for instance Merill I Skolniks 3 books Radar Systems 1962, Radar Hbk 1970 and Radar Hbk 1990—all from McGraw Hill). Typically a low radar frequency (up to 3 GHz) is used for long range surveillance systems (100 km and more) while a high radar frequency (10 GHz and higher), where rain etc. may limit the measuring range, are used for high precision navigation, fire control etc. at rather small distances (a few km).
In connection with level gauging in tanks there may be quite a number of disturbing echoes. The disturbing echoes from the surroundings, either if the echoes come from structures in a tank, or in the field of air traffic control, from ground echoes interfering with an echo from an aircraft, will limit the ability to detect or measure the desired echo among the many undesired ones. One standard solution to decrease the influence of such undesired interference for a pulse radar, as an example, is to let the transmitter frequency jump around within a band of a few 100 MHz to over 1 GHz. The use of these small frequency jumps within a radar band is normally called “frequency agility” (see chapter 9.7 in Skolniks Radar Hbk from 1990 or DK Bartons book “Frequency agility and diversity” from Artech House 1977). Multiple frequencies are also used in CW radars both as a frequency modulated CW radar, FMCW (using a range of frequencies) and multiple frequency CW radar, MFCW (for instance described in Skolniks book Radar Systems from 1962). MFCW radar has also been described for liquid level gauging (“Microwave surface level monitor” by Stanley Stuchley in IEEE trans. on industrial electronics, August 1971 and in patent document NO 831198). A frequency agility pulsed radar for level gauging is suggested in published patent document U.S. 2002/0020216.
For a radar level gauge used in a tank the situation is in many ways different from an outdoor radar and the following conditions should especially be mentioned.
A) The tank geometry and structure may give many disturbing echoes, whereby normally a narrow beam is desirable, but on the other hand the mounting possibilities in the tank (the size of the already existing mounting hole) generally limit the available space for the antenna, which preferably should be big to give a narrow microwave beam from the antenna.
B) Another important factor is that many liquids or tank conditions creates a foam layer on the liquid or a layer of dirt on the antenna. Due to the special dielectric properties of water, wet dirt, especially, may give a disastrous limitation of the propagation already at a few tenths of a mm wet dirt or a foam layer with the corresponding water content.
C) The frequency bands used for level gauging are for the time being around 6 GHz (related to the ISM band at 5.8 GHz), around 10 GHz and around 25 GHz (related to the ISM-band at 24.5 GHz). Nearly all radar level gauges are operated in closed metallic tanks and at low power so the leakage of electromagnetic power can be kept within current rules in spite of the need for a rather big bandwidth as compared to for instance the width of the ISM-bands.
D) Typical for a radar level gauge is further that the surface not always moves. The surface echo may thus be mixed with fixed echoes from tank structures and a possibly destructive interference at certain frequencies may persist for a long time. At a pulse radar, a way to decrease such effects is to use more than one frequency which is a closely related reason to use frequency agility in standard radars. As comparison an FMCW radar sweeps over a range of frequencies thus avoiding said problem.
The installation conditions (such as foaming state etc.) are in many cases not well known as the tank may be old or not possible to open due to high pressure or extreme temperature in the tank, poisonous content etc.
The very diversified needs have created a number of different radar level gauges on the market among them 3 groups of gauges using frequencies around 6, 10 and 25 GHz. In all three cases typically 10% bandwidth is used both in case of an FMCW system and a pulsed system (in which case the bandwidth is determined by the pulse length). To illustrate the range of differences the lobe-width for the same antenna diameter is around 4 times bigger at 6 GHz compared to 25 GHz, while the attenuation through the same layer of dirt or foam corresponds to 4 times longer measuring distance at 6 GHz as compared to 25 GHz, given the same sensitivity. With the same measure, the possible range at 6 GHz is around two times the range at 6 GHz, given the same layer of dirt and the same antenna size. A 25 GHz radar level gauge is in practical cases not usable in combination with dirt and foam. The actual conditions may in practical cases be hard to know in advance so a change of gauge to one using another frequency frequently occurs.
The problems mentioned may be compensated for by use of more than one radar level gauge, where each of them can measure at a different frequency band. Such a solution is of course not wanted as it makes the system expensive and many times not possible to install in a tank.
Document U.S. Pat. No. 5,659,321 shows an example of a radar level gauge system enabling level measurements on two different microwave frequencies. This is achieved by the use of a radar level module and a frequency converter for allowing the same radar level module to be used also at the higher frequency. The choice of the frequency band is made at the installation of the radar level gauge in the tank and makes it possible to adapt the radar characterics to the conditions prevailing in the tank, which depends on type of tank or of the kind of material being stored in the tank. The type of antenna installed is matched to the choiced frequency. It is not possible by means of the disclosed system to change the frequency continuously or to change the frequency in dependence of varying conditions in the tank during normal operation of the radar level gauge system.